Enlightened

The 'good' ending, and the trickier of the two - you spare the Dark Ones. It hinges on your hidden moral points: bank enough across the whole game (give to beggars, listen to NPC conversations, spare human enemies, explore the optional areas - see Generous, Metro dweller, Explorer) so that at the very end, on top of the tower, Artyom snaps out of the vision in time. When the moment comes, shoot the missile-guidance control to stop the strike. Too few points and the option never triggers. Mutually exclusive with If it's hostile, you kill it. in one run, so build morality from the very start on this playthrough.